Optical resonators such as silica micro-spheres and micro-toroids can support whispering gallery modes (WGMs), where light circulates near the resonator surface and is confined by the total internal reflection at the dielectric boundary. Such resonators can exhibit very high quality (Q) factors, since the resonator surface can maintain atomic level smoothness. The combination of high Q factors and small resonator volumes has led to a wide range of applications in sensing, optomechanics, nonlinear optics, and quantum optics. In this dissertation, we introduce a new type of whispering gallery resonators (WGRs) based on micro-droplets in an immiscible liquid-liquid system. Within such an all-liquid platform, it is possible to achieve highly nonlinear coupling between light and liquid that can potentially lead to single-photon level optical nonlinearity. Specifically, we experimentally characterize a droplet (D~500um) of index matching fluid submerged in the water as a high-Q optical resonator, where we use an optical fiber taper to couple light into the droplet through non-contact evanescent coupling. The highest Q-factor observed in the experiment is 2x10^7 which closely matches the upper limit of intrinsic Q-factor set by the material absorption. Given with such a high Q factor, the WGM can exert strong radiation pressure on the droplet interface, push it outward, increase the length of optical path, and produce a red-shift in WGM resonance. Our experimental results have found that the ratio of those resonance shifts and the optical power coupled into the resonator is approximately 60 fm/μW. The result closely matches to our steady-state estimation based on solving the coupled Maxwell-Navier-Stokes equation. To investigate the dynamic interplay of light and liquid, we develop a harmonic oscillator (HO) model to describe the time-domain behaviors of the coupled optofluidic system. We find a good agreement between theoretical predictions and our experimental data. The shift of WGM resonance can potentially be induced by thermal effects. To estimate the magnitude of thermal effects, we also investigate the thermally induced nonlinear behaviors of WGMs in a cylindrical fiber resonator (D~125um), where we change the mechanism of heat dissipation by changing the cladding material (e.g. air and water). For direct temperature measurements, we use a fiber optical resonator with a fiber Bragg grating (FBG) inscribed in the fiber core to observe temperature shifts induced by the high-Q WGMs. Our result shows that the temperature increase in the fiber resonator in the water is 0.13 C, whereas the fiber resonator in air shows ~4.5 C increase in temperature. Our results suggest that the relatively high thermal conductivity of water suppresses thermal nonlinearity by ~50 times, and that the red-shifts of WGMs can largely be attributed to radiation pressure effect. / Ph. D. / Optical resonators are used to confine incoming light and store its energy in a small volume. The quality of such resonators’ optical confinement is represented by quality factor (<i>Q</i>). Among different types of optical resonators, whispering gallery resonator (WGR) is well known for its high-<i>Q</i>, where strong optical confinement is achieved by the total internal reflection at the curved internal surface of spherical / cylindrical dielectric volume. The combination of high <i>Q</i> factors and small resonator volumes has led to a wide range of applications in sensing, optomechanics, nonlinear optics, and quantum optics. In this dissertation, we introduce a new type of WGR based on oil micro-droplet in water. Such an all-liquid platform enables highly nonlinear coupling between optical power and liquid matter that can potentially lead to optical nonlinearity at single-photon energy level. Specifically, we experimentally characterize an oil droplet (<i>D</i> ≈ 500 <i>um</i>) submerged in the water as a high-<i>Q</i> optical resonator, where we use a tapered optical fiber to inject optical power into the droplet. The highest <i>Q</i> of whispering gallery mode (WGM) observed in our experiment is 2×10⁷ and given with the high amplification of optical power in droplet, the WGM can exert strong radiation pressure on the droplet interface, push it outward, increase the length of optical path, and produce a red-shift in WGM resonance. Our experimental results have found that the ratio of those resonance shifts and the optical power coupled into the resonator is approximately 60 fm/<i>μ</i>W. The result closely matches to our steady-state estimation based on solving the coupled Maxwell-Navier-Stokes equation. To investigate the dynamic interplay of light and liquid, we develop a harmonic oscillator (HO) model to describe the time-domain behaviors of the coupled optofluidic system. We find a good agreement between theoretical predictions and our experimental data. The shift of WGM resonance can potentially be induced by thermal effects. To estimate the magnitude of thermal effects, we also investigate the thermally induced nonlinear behaviors of WGMs in a cylindrical fiber resonator (D ≈ 125 <i>um</i>), where we change the mechanism of heat dissipation by changing the media (e.g. air and water) surrounding the resonator. For direct temperature measurements, we use a fiber optical resonator with a temperature sensor equipped inside to observe temperature shifts induced by the high-<i>Q</i> WGMs. Our result shows that the temperature increase in the fiber resonator in the water is 0.13 °C, whereas the fiber resonator in air shows ~4.5 °C increase in temperature. Our results suggest that the relatively high thermal conductivity of water suppresses thermal nonlinearity by ~50 times, and that the red-shifts of WGMs can largely be attributed to radiation pressure effect.

Yes / This study presents an original asymmetric stepped-impedance resonator filter combined with meander coupled-line structures and enabling the realisation of finite transmission zeros (TZs) and the implementation of multi-band bandpass filters. The meander coupled sections (MCSs) tune the TZs and resonant frequencies: with higher-order spurious frequencies cancelled by the TZs, a single wideband with wide stopband from 1.18 to 1.84 GHz is possible. Furthermore, by positioning the finite TZs between the high-order spurious frequencies and adjusting the coupling strength between resonators, a quint-wideband filter can be realised, with centre frequencies of 1.19, 4.29, 5.43, 6.97, 9.9 GHz and fractional bandwidths of 31.9, 15.4, 15.8, 4.3, 39.2%, respectively. More importantly, two filters with single/quad-wideband performance can be realised by tuning the parameters of the MCS, and therefore they can be designed separately by using only one original structure. The triple-wideband filter is realised with the help of the asymmetric parallel uncoupled microstrip section. These filter structures enjoy the advantage of single/multi-band versatility, structure reusability and simplicity. The good in-band and out-of-band performance, low loss and simple structure of the proposed single/tri/quint-wideband filters make them very promising for applications in future multi-standard wireless communication. / European Union's Horizon 2020 research and innovation programme under Grant agreement H2020-MSCA-ITN-2016 SECRET-722424.

Yes / This article presents a miniaturized triple-band antenna for Internet of Things (IoT) applications. The miniaturization is achieved by using a split square ring resonator and half ring resonator. The antenna is fabricated on an FR4 substrate with dimensions of (33 × 22 × 1.6) mm3. The proposed antenna resonates at the frequencies 2.4 GHz, 3.7 GHz, and 5.8 GHz for WLAN and WiMax applications. The obtained −10 dB bandwidth for the three bands of the proposed antenna are 300 MHz, 360 MHz, and 900 MHz, respectively. The measured reflection coefficient values of the proposed antenna corresponding to each resonant frequency are equal to −14.772 dB, −20.971 dB, and −28.1755 dB, respectively. The measured gain values are 1.43 dBi, 0.89 dBi, and 1 dBi, respectively, at each resonant frequency. There is a good agreement between the measured and simulated results, and both show an omnidirectional radiation pattern at each of the antenna resonant frequencies that is suitable for IoT portable devices.

Yes / This work presents a combination of a cup-shaped monopole antenna and an E-shaped Multi-Mode Resonator (MMR) with the presence of a pair of PIN diodes and a varactor diode to form a compact reconfigurable communication filtering antenna for interweave Cognitive Radio (CR) systems. The proposed filtering antenna operates in the WiMAX band, and it is fabricated on an FR4 substrate with overall dimensions normalized to the wavelength ( λ o ) of the first resonant frequency (0.413λ o × 0.516λ o × 0.0165λ o ). The step and continuous tuning serve the secondary user of the WiMAX CR system to communicate in the absence of the primary users at modifiable resonant frequencies and data rates.When the PIN diodes are OFF, the filtering antenna operates with a fixed odd mode resonant frequency and tunable even mode resonant frequency. This state results in a tunable antenna bandwidth covering a maximum measured frequency range of 3.25-4.02~ GHz and a minimum measured range equal to 3.25-3.58~ GHz. The ON state of the PIN diodes eliminates the antenna matching at the even mode resonant frequency while keeping a strong matching at the odd mode resonant frequency. The resulted operational measured frequency range of the antenna in this state is fixed at 2.9-3.28~ GHz. The filtering antenna has acceptable gain values at the pass band of the E-shaped MMR with a maximum simulated gain value equal to 2.5~ dB and a measured maximum gain equal to 2.48~ dB. The simulated and measured power patterns of the antenna for all diodes states are omnidirectional, which are convenient for portable CR gadgets. / This work was supported in part by the Innovation Programme under Grant H2020-MSCA-ITN-2016 SECRET-722424, and in part by the U.K. Engineering and Physical Sciences Research Council (EPSRC) under Grant EP/E022936/1.

PM2.5 particle sensing has been done using surface acoustic wave based sensor for two different frequencies. Due to mass loading and elasticity loading on the sensor's surface, the center frequency of the sensor shifts. The particle concentration can be tracked based on that frequency shift. The fit factor test has been conducted using higher frequency SAW sensor. The consist results has been achieved for particle sensing and fit factor test with SAW based sensor.

This thesis presents the design, modeling, and experiment of a novel pressure sensor using a dual-mode AT-cut quartz crystal resonator with beat frequency analysis based temperature compensation technique. The proposed sensor can measure pressure and temperature simultaneously by a single AT-cut quartz resonator. Apart from AT-cut quartz crystal, a newly developed Langasite (LGS) crystal resonator is also considered in the proposed pressure sensor design, since LGS can operate in a higher temperature environment than AT-cut quartz crystal. The pressure sensor is designed using CAD (computer aided design) software and CAE software - COMSOL Multiphysics. Finite element analysis (FEA) of the pressure sensor is performed to analyze the stress- strain of the sensor's mechanical structure. A 3D printing prototype of the sensor is fabricated and the proposed sensing principle is verified using a force-frequency analysis apparatus. Next to the 3D printing model verification, the pressure sensor with stainless steel housing has been fabricated with inbuilt crystal oscillator circuit. The oscillator circuit is used to excite the piezo crystal resonator at its fundamental vibrational mode and give the frequency as an output signal. Based on the FEA and experimental results, it has been concluded that the maximum pressure that the sensor can measure is 45 (psi). The pressure test results performed on the stainless steel product shows a highly linear relationship between the input (pressure) and the output (frequency).

Optical microcavities (OMs) are receiving increasing attention owing to their potential applications ranging from cavity quantum electrodynamics, optical detection to photonic devices. Recently, rolled-up structures have been demonstrated as OMs which have gained considerable attention owing to their excellent customizability. To fully exploit this customizability, asymmetric and topological rolled-up OMs are proposed and investigated in addition to conventional rolled-up OMs in this thesis. By doing so, novel phenomena and applications are demonstrated in OMs. The fabrication of conventional rolled-up OMs is presented in details. Then, dynamic mode tuning by a near-field probe is performed on a conventional rolled-up OM. Next, mode splitting in rolled-up OMs is investigated. The effect of single nanoparticles on mode splitting in a rolled-up OM is studied. Because of a non-synchronized oscillating shift for different azimuthal split modes induced by a single nanoparticle at different positions, the position of the nanoparticle can be determined on the rolled-up OM. Moreover, asymmetric rolled-up OMs are fabricated for the purpose of introducing coupling between spin and orbital angular momenta (SOC) of light into OMs. Elliptically polarized modes are observed due to the SOC of light. Modes with an elliptical polarization can also be modeled as coupling between the linearly polarized TE and TM mode in asymmetric rolled-up OMs. Furthermore, by adding a helical geometry to rolled-up structures, Berry phase of light is introduced into OMs. A -π Berry phase is generated for light in topological rolled-up OMs so that modes have a half-integer number of wavelengths. In order to obtain a deeper understanding for existing rolled-up OMs and to develop the new type of rolled-up OMs, complete theoretical models are also presented in this thesis.

The light-matter interaction in the near-field of high-Q resonators in SOI and SiN platforms is studied. The interactions of high-Q traveling-wave resonators with both resonant and non-resonant nanoparticles are studied and different applications based on this enhanced interactions in near-field such as high-resolution imaging of mode profile of high-Q resonators, label-free sensing, optical trapping, and SERS sensing are investigated. A near-field imaging system for the investigation of the near-field phenomena in the near-field of high-Q resonators is realized. A new technique for high-resolution imaging of the optical modes in high-Q resonators based on the near-field perturbation is developed that enables to achieve a very high resolution (< 10 nm) near-field image. The prospect of the high Q resonators on SOI platform for highly multiplexed label-free sensing and the effect of different phenomena such as the analyte drift and diffusion and the binding kinetics are studied. Also, the possibility of enhancing nanoparticle binding to the sensor surface using optical trapping is investigated and the dynamic of a nanoparticle in the high-Q resonator optical trap is studied. Furthermore, the interaction between a resonant nanoparticle with a high-Q microdisk resonator and its application for SERS sensing is studied. A model for interaction of resonant nanoparticles with high-Q resonators is developed and the optimal parameters for the design of coupled microdisk resonator and a plasmonic nanoparticle are calculated. The possible of resonant plasmonic nanoparticle trapping and alignment in an SiN microdisk resonator optical trap is also shown.

Through continued innovation and growth, traveling wave tube amplifiers (TWTAs) remains the microwave power amplifiers of choice in a wide range of high power microwave and millimeter-wave applications specifically for aerospace applications with the volume, weight, bandwidth and power constraints. These advances can be credited to device innovation, improved modeling and design and development of advanced materials and construction techniques. This thesis aims at advancing the present technology of TWTs with coupled resonator slow-wave structures (SWSs) by a combination of device innovation, development of enhanced analytical and field analysis codes and understanding gained through improved modeling, simulation and experimentation. In a TWT, the SWS that slows the RF wave velocity down to near the electron beam velocity for interaction with the electron beam primarily determines the microwave performances of the tube. As compared to helix SWS, the coupled resonator SWS is capable of handling high peak and average powers with higher efficiency and TWTs based on these SWS are well suited for air-borne or space-borne radar systems and the major focus of this thesis is on the analysis and design of coupled resonator SWSs. As a part of this thesis, improved analytical codes based on quasi-TEM analysis and equivalent circuit analysis have been developed. The technical formulation is explained and the improvements made for enhanced accuracy and for incorporation of different types of coupled resonator SWSs detailed. Using these models new variants of coupled resonator SWSs have been investigated. The SWSs proposed are the ladder-core inverted slot mode SWS and the inductively loaded inter digital SWS (ILID-SWS). The possibility of achieving both coalesced mode design that gives wide bandwidth and multi beam design that improves the peak power and gain using rectangular ILID-SWS is presented. The properties of these proposed SWSs have been compared with the existing SWSs and found to give superior performance. Also an improved modeling and simulation technique using 3-D electromagnetic codes has been proposed and the conventional cold test measurement procedure has been modified for more accurate results. Numerous illustrative examples are presented throughout the thesis highlighting the analytical model and simulation code validation with experimental results. The experimentations have been carried out on the real SWS model that have been fabricated and assembled. Further, the contribution of the thesis is towards the development of a field analysis model for analysis of a corrugated waveguide SWS, based on the coupled integral equation technique (CIET), which is a combination of mode matching technique (MMT) and method of moments. The technical formulation and computational methodology employed in the model are explained and some of the most important aspects of implementation like the handling of singularities and choice of parameters controlling the accuracy is discussed. The accuracy and speed of the code is demonstrated by comparing CIET with MMT and 3-D electro magnetic simulators based on finite difference time domain (FDTD) method and finite element method (FEM). The CIET code developed is quite faster than the existing numerical methods and helps in solving the convergence problem associated with the MMT.

Realising Si-based electrically driven light emitters in a process technology compatible with mainstream microelectronics CMOS technology is key requirement for the implementation of low-cost Si-based optoelectronics and thus one of the big challenges of semiconductor technology. This work has focused on the development of microcavity enhanced silicon LEDs (MCLEDs), including their design, fabrication, and experimental as well as theoretical analysis. As a light emitting layer the abrupt pn-junction of a Si diode was used, which was fabricated by ion implantation of boron into n-type silicon. Such forward biased pn-junctions exhibit room-temperature EL at a wavelength of 1138 nm with a reasonably high power efficiency of 0.1%. Two MCLEDs emitting light at the resonant wavelength about 1150 nm were demonstrated: a) 1-lambda MCLED with the resonator formed by 90 nm thin metallic CoSi2 mirror at the bottom and semitransparent distributed Bragg reflector (DBR) on the top; b) 5.5-lambda MCLED with the resonator formed by high reflecting DBR at the bottom and semitransparent top DBR. Using the appoach of the 5.5-lambda MCLED with two DBRs the extraction efficiency is enhanced by about 65% compared to the silicon bulk pn-junction diode.